1. Field of the Invention
The present invention is in the field of producing cement or the like from a raw material utilizing powdered coal as the principal fuel. The specific improvements of the present invention involve treating an exhaust gas fraction which normally contains a relatively high concentration of injurious substances to cool the same, removing the injurious substances from the cooled gas fraction, and then passing the resulting gas back into the drying stage for the powdered coal to recover much of its heat content.
2. Description of the Prior Art
A system is shown in German OS No. 2,745,424 in which a coal grinding and drying system is installed in the area of a raw meal pre-heating system comprising part of a burning system for the manufacture of cement clinker. The coal grinding and drying system generates a coal dust which is used as a fuel in the burning system. The coal grinding and drying system is fed with an exhaust gas stream taken from the pre-heating stage of the burning system, the partial exhaust gas stream having an oxygen content of approximately 5% maximum making it well suited for use as a drying gas because it is sufficiently inert for risk-free operation of the coal grinding and drying system.
When the material to be treated such as raw cement meal contains alkali compounds, chlorine compounds, sulfur compounds and similar injurious substances, the exhaust gases which result must be removed by means of a bypass at a location prior to the burning system exhaust gas before the exhaust gas is employed for pre-heating the raw meal or for raw meal calcining in order to avoid the circulation of injurious substances in the pre-heating and burning system. Such bypass gas must be very quickly cooled from approximately 1100.degree. through 1300.degree. C. to about 400.degree. to 600.degree. C. by mixing it directly with cold air before its removal through pipe lines and before it is cleansed and subsequently discarded because the injurious substances would otherwise lead to caking in gas-conducting pipe lines. It is evident that the bypass gas withdrawn from the burning system becomes so greatly enriched with oxygen after being mixed with cold air generally containing 21% oxygen that it is no longer suitable for the inert operation of the coal grinding and drying system. Moreover, with today's energy prices and those which can be expected in the future, it is no longer justifiable from the standpoint of heat economy to discard rotary kiln exhaust gases from the burning system which contain such injurious substances without attempting recovery of some of their heat content.